Presently, in the field of submarine optical communications, a Command & Response (C&R) monitoring system is generally used to monitor submarine transmission equipment such as a relay station (repeater) and a branch unit. The C&R monitoring system adopts the working mode of direct communication, in which terrestrial equipment sends a monitoring signal to submarine transmission equipment to be monitored, and the submarine transmission equipment feeds back a reply signal of working conditions and various parameters in the current state after receiving the command or at a predetermined trigger condition.
In the prior art, the method for modulating information sent by the C&R monitoring system includes: modulating a sub-carrier scramble signal with a smaller pilot tone modulation depth on a main optical signal transmitted on a main optical channel, and performing Amplitude Shift Keying (ASK) modulation on the sub-carrier scramble signal, so as to carry a monitoring signal or a reply signal. The pilot tone modulation refers to superpose a low-frequency sinusoidal signal with small amplitude on an optical signal transmitted by a transmitter to serve as an identifier; and detect sinusoidal signals of various frequencies to identify power levels of corresponding optical signals through the identifier at relay stations, so as to extract desired information. The pilot tone modulation depth of the scramble signal is generally smaller than 10% of the amplitude value of the main optical signal transmitted on the main optical channel, so that the transmission of normal services is not influenced.
The relay station in a submarine optical communication system generally adopts the Automatic Level Control (ALC) mode, so that the output optical power of the main optical signal transmitted on the main optical channel can be maintained at a fixed level. The scramble signal may be modulated on pump light of an Erbium Doped Fiber Amplifier (EDFA), so as to modulate the scramble signal on the main optical signal transmitted. However, if the output optical power input to the relay station changes or the ambient temperature changes (output of a laser is influenced by the ambient temperature), and the strength of the scramble signal is not adjusted, the pilot tone modulation depth of the scramble signal modulated on the pump light changes, which results in that the pilot tone modulation depth of the scramble signal on the main optical channel changes with the change of the input optical power. Excessive large pilot tone modulation interferes main optical signals actually transmitted on the main optical channel, and excessive small pilot tone modulation depth influences information transmission of the C&R monitoring system.
In the prior art, a method for fixing a pilot tone modulation depth is adopted, in which according to the strength of a pump current, the amplitude of a scramble signal is adjusted, so that a ratio of the amplitude of the scramble signal to the amplitude of the pump current is constant, thereby achieving the purpose of fixing the pilot tone modulation depth of the scramble signal on a main optical channel. FIG. 1 is a schematic structural diagram of a relay station with a fixed pilot tone modulation depth in the prior art. As shown in FIG. 1, the relay station includes an uplink control part 10 and a downlink control part 10′, in which components of corresponding numerals included in the uplink control part 10 and the downlink control part 10′ are the same component in the relay station, for example, an adjustable gain amplifier 107 and an adjustable gain amplifier 107′.
In view of the uplink control part 10, an optical signal, including a main optical signal and a monitoring signal modulated on the main optical signal, enters an erbium doped fiber 101 from a transmission link 100; pump light generated by a pump Laser Diode (LD) 106 is reflected into the erbium doped fiber 101 by a coupler 102, and the incident optical signal is amplified; most of the amplified optical signal is continuously transmitted on the transmission link 100 when passing through an optical splitter 103, and the rest small part of optical signal enters a photodiode (PD) 104 and is converted to a current signal, in which this part of current signal serves as a feedback, and is finally used to adjust the pump light output by the LD 106, so as to compensate the optical signal output by the coupler 102. A Direct Current (DC) signal of the current signal partially enters an output light stabilization circuit 105, the output light stabilization circuit 105 compares the current value of the DC signal with a preset value and generates a control current IDC. The control current IDC is used to control the strength of the pump light, and enables the bump light to be coupled on the output optical signal, so as to stabilize the optical power of the output optical signal, that is, to maintain the output optical power of the optical signals output on the main optical channel at a fixed level. An Alternating Current (AC) in the current signal partially enters a bandpass filter 108 with a central frequency of f1, in which f1 is a carrier frequency of the monitoring signal. The bandpass filter 108 merely permits the carrier of the monitoring signal to enter a detection control unit 109. The carrier of the monitoring signal is demodulated to obtain the monitoring signal, and the parameters such as input/output optical power and laser temperature are detected according to indication of the monitoring signal. Afterwards, a reply signal is formed and is modulated in a sinusoidal signal generated by an oscillation circuit 111. Then, the signal is output to an adjustable gain amplifier 107 from the detection control unit 109, in which the sinusoidal signal on which the reply signal is modulated is amplified by the adjustable gain amplifier 107, and an AC signal IAC is output to the LD 106. The AC signal IAC and the IDC are combined and then sent to the pump LD 106, so as to drive the pump LD to generate pump light with disturbance being the same as the AC signal IAC. An output optical signal enters the erbium doped fiber 101 from the coupler 102, and when the output optical signal is amplified with the pump light, a reply signal is modulated on the output optical signal through the disturbance of the pump light and is sent back to the terrestrial monitoring unit for detection.
Before the IAC and the IDC are combined, in order to adjust the AC signal IAC on which the reply signal is modulated, a part of the IDC is sent to an operational amplifier 110 and converted into a voltage signal. The voltage signal is used to control the gain of the adjustable gain amplifier 107, so that the amplitude value of the AC signal IAC changes with the IDC at a fixed proportion finally, thereby achieving the purpose of fixing the pilot tone modulation depth of the reply signal on the main optical channel.
FIG. 2 is a curve diagram of the strength of pump light and an output gain of the LD in the relay station in FIG. 1. As shown in FIG. 2, the relation of the output gain of the EDFA and the strength of the pump light output by the LD is expressed as a monotonic curve, and the slope of the monotonic curve decreases with the increase of the strength of the pump light. It can be seen that when the strength of the pump light increases, the disturbance on the pump light may cause a disturbance decreasing on the output light. If the input optical power input to the relay station has a severe jitter, the severe jitter responded on the pump light may not be represented on the output gain of the EDFA, so that it cannot be ensured that the pilot tone modulation depth of the scramble signal on the main optical channel is fixed.